As disclosed in conventional documents including U.S. Pat. Nos. 3,869,492 and 3,869,430, wholly aromatic polyamide filaments are usually formed by a method comprising the steps of: preparing wholly aromatic polyamide polymer by polymerizing aromatic diamine and aromatic diacid chloride in a polymerization solvent including N-methyl-2-pyrrolidone; dissolving the prepared polymer in a concentrated sulfuric acid solvent to prepare a spinning solution; forming filaments by passing the spinning solution through a spinning nozzle (40) to obtain a spun material and passing the spun material through a non-coagulative fluid layer into a coagulating solution bath (50); and washing, drying and heat treating the formed filaments.
FIG. 2 is a schematic view illustrating a conventional process for production of wholly aromatic polyamide filaments by a normal dry wet spinning method.
With conventional processes for preparation of a spinning dope, two or more aramid polymers are usually fed into an extruder for preparation of the spinning dope at the same time. Accordingly, the above processes have a requirement that each of the aramid polymers should have a constant particle diameter ranging from 75 to 850 μm and an inherent viscosity (IV) of not less than 5.5 and, more preferably, not less than 6.0.
More particularly, if the aramid polymer has the particle diameter of not more than 75 μm or the inherent viscosity of less than 5.0, the aramid polymer becomes extremely brittle by a sulfuric acid solvent although it is dissolved well in the solvent, thereby leading to significant reduction of strength, especially, side impact strength of aramid filaments in manufacturing the same.
Furthermore, when both of the aramid polymers having relatively larger and smaller particle diameters are simultaneously fed into the extruder for preparation of the spinning dope according to the conventional process, a retention time for each of the aramid polymers in the extruder is extended according as the aramid polymer having the larger particle diameter is completely dissolved in the sulfuric acid solvent. As a result, the aramid polymer having the smaller particle diameter becomes very brittle caused by the sulfuric acid solvent, thereby causing the resultant spinning dope with excessively reduced inherent viscosity (IV) unsuited for manufacturing high strength aramid filaments. In case of introducing the aramid polymers having different IVs into the extruder at once, there is also a problem of extreme brittleness of the polymers by the same reason as described above.
Therefore, the aramid polymers with IVs of not more than 5.0 and/or particle diameters of not more than 75 μm have been used in manufacturing aramid pulp products of low price.
In addition, even if the aramid polymer with IV of 5.0 to 5.5 was adopted, there was only a slight reduction in strength of aramid filaments formed using the aramid polymer.
As described above, the conventionally known processes have the problem in that the aramid polymers having too small or large particle diameters and/or IVs of less than 5.5 cannot be employed in production of aramid filaments, thereby causing increase of production cost thereof.
The present invention has a purpose of solving the above problems and provides a method of manufacturing high strength aramid filaments by using aramid polymers with constant particle diameters and/or IVs out of proper ranges.